Field of the Invention
The invention relates generally to hard disk drive memory storage devices for computers. More particularly it relates to disk drive apparatus and to a method for writing servotrack information therein. More specifically it relates to alleviating the need for a complex mechanical and/or optical positioning system to establish servopatterns on the recording surfaces of the recording media.
General Background Art Relating to Self-Servowriting
As described in International Patent Application, WO 94/11864, increased levels of storage capacity in floppy and hard disk drives are a direct result of the higher track densities possible with voice-coil and other types of servo positioners as well as the ability to read and write narrower tracks by using, for example, magnetoresistive (MR) head technology. Previously, low track density disk drives were able to achieve satisfactory head positioning with leadscrew and stepper motor mechanisms. However, when track densities are so great that the mechanical error of a leadscrew-stepper motor combination is significant compared to track-to-track spacing, an embedded servo is needed so that the position of the head can be determined from the signals it reads.
Conventional hard disk manufacturing techniques including writing servotracks on the media of a head disk assembly (HDA) with a specialized servowriter instrument. Laser positioning feedback is used in such instruments to read the actual physical position of a recording head used to write the servotracks. Unfortunately, it is becoming more and more difficult for such servowriters to invade the internal environment of a HDA for servowriting because the HDAs themselves are exceedingly small and depend on their covers and castings to be in place for proper operation. Some HDAs are the size and thickness of a plastic credit card. At such levels of microminiaturization, traditional servowriting methods are inadequate.
Conventional servo-patterns typically comprise short bursts of a constant frequency signal, very precisely located offset from a data track's center line, on either side. The bursts are written in a sector header area, and can be used to find the center line of a track. Staying on center is required during both reading and writing. Since there can be between seventeen to sixty, or even more, sectors per track, that same number of servo data areas must be dispersed around a data track. These servo-data areas allow a head to follow a track center line around a disk, even when the track is out of round, as can occur with spindle wobble, disk slip and/or thermal expansion. As technology advances provide smaller disk drives, and increased track densities, the placement of servo data must also be proportionately more accurate.
Servo-data are conventionally written by dedicated, external servowriting equipment, and typically involve the use of large granite blocks to support the disk drive and quiet outside vibration effects. An auxiliary clock head is inserted onto the surface of the recording disk and is used to write a reference timing pattern. An external head/arm positioner with a very accurate lead screw and a laser displacement measurement device for positional feedback is used to precisely determine transducer location and is the basis for track placement and track-to-track spacing. The servo writer requires a clean room environment, as the disk and heads will be exposed to the environment to allow the access of the external head and actuator.
U.S. Pat. No. 4,414,589 to Oliver et al. teaches servowriting wherein optimum track spacing is determined by positioning one of the moving read/write heads at a first limit stop in the range of travel of the positioning means. A first reference track is then written with the moving head. A predetermined reduction number or percentage of amplitude reduction X %, is then chosen that is empirically related to the desired average track density. The first reference track is then read with the moving head. The moving head is then displaced away from the first limit stop until the amplitude of the first reference track is reduced to X % of its original amplitude. A second reference track is then written with the moving head and the moving head is then displaced again in the same direction until the amplitude of the second reference track is reduced to X % of its original value. The process is continued, writing successive reference tracks and displacing the moving head by an amount sufficient to reduce the amplitude to X % of its original value, until the disc is filled with reference tracks. The number of reference tracks so written is counted and the process is stopped when a second limit stop in the range of travel of the positioning means is encountered. Knowing the number of tracks written and the length of travel of the moving head, the average track density is checked to insure that it is within a predetermined range of the desired average track density. If the average track density is high, the disc is erased, the X % value is lowered and the process is repeated. If the average track density is low, the disc is erased, the X % value is increased and the process is repeated. If the average track density is within the predetermined range of the desired average track density, the desired reduction rate X %, for a given average track density, has been determined and the servo writer may then proceed to the servo writing steps.
Unfortunately, Oliver et al. do not disclose how to generate a clock track using the internal recording data heads, as this is achieved by an external clock head. Oliver also do not teach how to determine the track spacing during propagation. This results in the requirement of writing an entire disk surface and counting the number of written tracks to determine the track spacing. Further, Oliver et al. do not examine the variation in the plurality of heads with the disk drive to set the track pitch accordingly. Finally, Oliver et al. do not teach how to limit the growth of errors during the radial propagation growth.
As also described in International Patent Application WO94/11864, a method for writing a servo-pattern with a disk drive's own pair of transducers is described in U.S. Pat. No. 4,912,576, issued Mar. 27, 1990 to Janz. Three types of servo-patterns are used to generate three-phase signals that provide a difference signal having a slope that is directly proportional to velocity. Servo-patterns that are substantially wider radially than the nominal track-to-track separation are possible. This helps improve readback amplitudes, and thus servo performance. Janz observes that the signal level from a transducer is a measure of its alignment with a particular pattern recorded on the disk. If the flux gap sweeps only forty percent of a pattern, then the read voltage will be forty percent of the voltage maximum obtainable when the transducer is aligned dead-center with the pattern. Janz uses then phenomenon to straddle two of three offset and staggered patterns along a centerline path intended for data tracks.
In a preferred process, Janz reserves one side of a disk for servo and the other side for data. The disk drive includes two transducers on opposite surfaces that share a common actuator. To format an erased disk for data initialization, a first phase servo is written on the servo side at an outer edge. The transducers are then moved-in radially one half of a track, as indicated by the first phase servotrack amplitude, and a first data-track is recorded on the data side. The transducers are again moved-in radially one half of a track, this time as indicated by the first data-track amplitude, and a second phase servotrack is recorded on the servo side. The transducers are again moved-in radially one half of a track, as indicated by the second phase servotrack amplitude, and a second data-track is recorded on the data side. The transducers are moved-in radially another one half of a track, as indicated by the second data-track amplitude, and a third phase servotrack is recorded on the servo side. The transducers are moved-in radially one half of a track, as indicated by the third phase servotrack amplitude, and a third data-track is recorded on the data side. This back-and-forth progress is repeated until the entire two surfaces are written. If too few or too many tracks were thus written, the disk is reformatted once more, but with a slight adjustment to stew inward slightly more or slightly less than one-half a track width, as appropriate. Once the disk drive has been formatted with an entire compliment of properly spaced servotracks, the data-tracks have served their purpose and are erased in preparation for receiving user data.
Unfortunately, the method described by Janz consumes one entire disk surface for servotracks and requires two heads working in tandem. Track-to-track bit synchronism is also not controlled, and seek times to find data between tracks would thus be seriously and adversely impacted. Transducer flying height variations and spindle runout that occur within a single revolution of the disk, and media inconsistencies can and do corrupt radial position determinations that rely on a simple reading of off-track read signal amplitudes. Prior are methods are inadequate for very high performance disk drives.
IBM Technical Disclosure Bulletin, Vol. 33, No. 5 (October 1990) entitled "Regenerative Clock Technique For Servo Track Writers" suggests servo writing of a head/disk assembly after the covers are in place by means of the product head and without the use of an external position encoder disk. A single clock track is written at the outer diameter and divided into alternate A and B phrases. The head is then stepped inwards half a track at a time using each phase alternately as a source of clock information from which servo information in the servo sectors preceding each data field and further clock signals in the alternate phase can be written. The half track steps ensure that the previously written clock information can be read. The technique dispenses with a dedicated servo writer clock head and associated mechanisms.
International Patent Application WO94/11864 teaches a hard disk drive comprising a rotating disk with a recording surface, a transducer in communication with the surface and servo-actuator means for radially sweeping the transducer over the surface, a variable gain read amplifier connected to the transducer, an analog to digital converter (ADC) attached to the variable gain amplifier, an erase frequency oscillator coupled to the transducer for DC erasing of the disk surface, a memory for storing digital outputs appearing at the ADC, and a controller for signaling the servo-actuator to move to such radial positions that result in transducer read amplitudes that are a percentage of previous read amplitudes represent in the digital memory. Bit-synchronism between tracks is maintained by writing an initial clock track with closure and then writing a next clock track including a regular sequence of clock bursts a half-track space offset such that the initial clock track can be read in between writing clock bursts and the read signal is used to frequency-lock an oscillator which is used as a reference for the writing of clock bursts of the next track. A checkerboard pattern of clock bursts is thus created. All subsequent tracks are built incrementally by stepping off a half of a track from the last track written, which comprises clock bursts, and writing a next new sequence of clock bursts that interlace with the previous track's clock bursts.
Background Art Specific to Radial Self-Propagation
The process of disk file servowriting using only the internal recording transducer and product actuator, referred to as self-servowriting, involves a combination of three largely distinct sub-processes, writing and reading magnetic transitions to provide precise timing, positioning the recording transducer at a sequence of radial locations using the variation in readback signal amplitude as a sensitive position transducer, and writing the actual product servopattern at the times and radial locations defined by the other two processes. The present invention addresses significant shortcomings of the radial positioning process, referred to here as self-propagation, as previously described in the prior art, specifically U.S. Pat. No. 4,414,589 by Oliver, et al., International Patent Application WO 94/11864 by Cribbs et al., as well as the above mentioned related U.S. patent application Ser. No. 08/028,044 by Chainer et al. The concept of self-propagation as applied to disk file servowriting, while promising very substantial benefits with regard to servowriting cost (as pointed out in the 1983 U.S. Pat. No. 4,414,589 by Oliver, et al. for example), has not yet been commercially realized.
Briefly, the shortcomings in previously described techniques are associated with lesser accuracy in the placement of the servopatterns as compared with conventional servowriting. The requirements for ever closer track spacing in disk files makes highly accurate servopattern writing a necessity. The cost advantages of self-propagation are not sufficient to supplant conventional servowriting without addressing and solving the problem of servopattern inaccuracy. Two factors contribute to reduced servopattern accuracy when using self-propagation; error compounding and higher levels of random mechanical motion. In conventional servowriters the radial positioner is an external device that affords stable location of the recording transducer by virtue of its relatively high mass and stiff attachment to a large granite block that has minimal vibration. Random mechanical motion of the recording transducer is therefore kept very small, and the track shapes defined by the servopatterns are almost perfectly circular. Errors that do occur are totally uncorrelated from track to track, so compounding is never a consideration. Average track to track spacing is accurately maintained through the use of a laser displacement measurement device. In self-propagation, the radial position signal that is used to servo-control the actuator is derived from measurements of the readback amplitude of patterns that were written during a previous step. An error in one step of the process can affect the position of the recording transducer on the next step so it is essential that the compounding effects of a very large number of steps be considered.
A simple solution is to use only weak servo control so that radial placement errors are averaged out rather than dynamically tracked. This is the approach described in Chainer et al. This is also implicit in the patent of Oliver et al., where the propagation pattern is physically overwritten at each step. This means that the readback amplitude cannot be determined at the time of writing, hence the servo controller must be essentially free running with no ability to dynamically adjust to the pattern. However, random mechanical motion may be kept small only by using a very tight servo control. Thus, elimination of error compounding comes at the expense of higher random mechanical motion, thus making this solution unattractive. Also, the use of a low bandwidth servo requires long times for stepping and settling to the proper location, leading to increased servowrite times and higher cost.
In Cribbs et al. there are suggestions that the servo control does dynamically track the written pattern edges, but there is no discussion of how this affects error compounding. In fact, they describe a refinement to reduce "hunting" and "dithering" of the actuator that most likely arises from just such a compounding effect. In further discussion below it will become apparent that this refinement merely hides the presence of excessive error compounding during the servowrite process, rather than actually eliminating it.
Servopattern errors of different types have varying degrees of importance with regard to ultimate disk file performance. The absolute radial position of each track on the disk needs to be controlled only moderately well since regular updates of track count are available, even during high speed seeking between tracks that are far apart. Similarly the average track spacing in absolute units is not especially tightly constrained. There is a maximum absolute spacing such that the desired number of data tracks be contained between the inner and outer mechanical stops of the actuator, but as long as the recording transducers of the disk file are narrow enough the spacing could be less than this maximum with no ill effects. Thus, it is not the absolute spacing that is critical, but rather the relative spacing as compared to the recording transducer. The techniques described in Chainer et al. for determining the widest head within a disk file and using measurements from that head to set the track spacing for all heads are generally effective for ensuring that the average track spacing meets the necessary criteria. However an unforeseen problem with regard to the determination of the ideal amplitude reduction factor to use for a servo control reference during self propagation has arisen with the introduction of recording transducers in which the read and write elements do not coincide. A need exists for a method to compensate for misalignment of these two elements such as arises from variations in normal manufacturing, as well as changes in their relative alignment with respect to disk tracks when a rotary form of actuator is used to position the recording transducer.
While it is desirable that the track shapes be reasonably close to circular in shape, the disk file servoactuator will repeatedly follow moderate amounts of deviation so that data tracks will be read back on the same trajectory as they were written. Thus, as long as adjacent tracks are distorted similarly, absolute circularity need only be maintained within fairly coarse bounds, determined by a desire to limit the repeatable motion of the actuator to roughly one head width or so, as opposed to readback mis-registration concerns which require a limit of a small fraction of the head width.
The most important consideration for servopattern accuracy is local track to track spacing, referred to as track squeeze, since a prime requirement in disk files is that adjacent tracks be everywhere separated by some minimum spacing. This ensures that adjacent track information will not be detected on readback (this causes data read errors) and, even more importantly, that adjacent track data will never be overlapped excessively during writing since this could result in permanent loss of user data. Track squeeze is determined by the radial separation between adjacent track locations as defined by the product servopattern written on each track and at each angular location around the disk. In other words, the detailed shape of each track relative to its neighbors must be considered, not just the track to track distance averaged around the whole disk. This is because the servo-control of the actuator during actual file operation is capable of following distortions from perfect circularity and will produce mishappen data tracks. The data tracks do not exactly match the servopattern track shapes because the servo loop follows accurately only up to a limited frequency, but it is a reasonably good approximation to simply take them as being identical. The general arguments that follow are unaffected by this level of detail, but one would wish to include this effect when determining a precise product specification for track squeeze.
In setting the minimum allowable spacing, one must take into account the existence of random fluctuations about the desired track location (as defined by the servopattern) that result from mechanical disturbances during actual file operation. One of the largest sources of disturbance is the turbulent wind blowing against the actuator from the spinning disks. The total amount of fluctuation, referred to as TMR (for track mis-registration), defines a relevant scale for judging the required accuracy of servopattern placement. If servopattern errors are roughly equal to or greater than the TMR then a substantial fraction of the track spacing margin will be required as compensation, leading to a reduction in total disk file data capacity. Once the servopattern placement errors are less than about half the TMR, however, further reduction does not provide much improvement in total data capacity. The random mechanical motion that results when a very low bandwidth servo is used, is observed to be roughly 5 times greater than the TMR experienced during file operation. Clearly, the use of such a servo loop during self-propagation would result in unacceptably large errors.
Self-propagation patterns consist of bursts of transitions located at intervals around the disk surface. The edges of the bursts comprise a set of points that define a track shape that the servo controller will attempt to follow on the next step of the process. Thus, errors in the transducer position during the writing of the bursts appear as distortions away from a desired circular track shape when the actuator is subsequently moved outward to servo off the edges of the bursts. Sensing this non-circular trajectory during the next burst writing step, the servo controller moves the actuator in an attempt to follow it. This causes the new bursts to be written at locations that reflect (via the closed-loop response of the servo loop) the errors that were present on the preceding step together with additional errors arising during the present step. Each additional step in the process therefore carries forth a "memory" of all preceding track shape errors. This "memory" depends on the detailed closed-loop response of the servo loop. Effects that result in track shape errors include random mechanical motion as well as modulation in the width of the written track that may come from variations in the properties of the recording medium or in the flying height of the transducer. These modulation effects are typically small compared to the total data track width but are often very repeatable from track to track and can grow to very substantial levels if compounded repeatedly. Uncontrolled growth of such errors can lead to excessive amounts of absolute track non-circularity. In some cases error compounding can lead to exponential growth of errors. All error margins will then be exceeded, and the self-propagation process itself will likely fail.
In Cribbs et al. written track width modulation arising from flying height variations is described as a source of track shape error that impacts the self-propagation process. A procedure is outlined in which three extra revolutions of the disk are used to smooth the servo error control signals so as to reduce "hunting" and "dithering" of the servo actuator before each step of writing propagation bursts. It is unlikely that track width modulation large enough to detect excessive "hunting" could occur within any one step of burst writing, especially since width modulation is a secondary effect as compared to on-track readback modulation, and a preliminary step in their process is to reject all disk files having excessive on-track modulation. It is more likely that, in accordance with our experiences and detailed analysis, intrinsic width modulation typically appears at the level of only a few percent of the track width, but grows through error compounding to much larger levels. It is also clear that a signal that is discernible in the position error signal of a high gain servo loop is indicative of an underlying track shape error that is far greater than the error signal itself. This follows from the fact that the position error signal is merely the residual part of the underlying track shape error that the servo loop was unable to follow. The procedure of adjusting the target amplitudes while track following so as to smooth the position error signal is one in which the underlying track shape error is merely hidden, not eliminated. Below, we show that the detailed response of the servo loop is critical to understanding the problem of error compounding. Adjustments of the target amplitudes as described by Cribbs et al. may work to limit error growth with some types of servo loops, but since no specification of servo response is given, the issue is left to chance. Even if the smoothing were to work, the solution is unattractive in that three extra revolutions of the disk are required at each step in the process. This doubles the servowrite time, and raises the cost.
As mentioned above, self-propagation suffers from higher levels of random mechanical motion than conventional servowriters having massive external positioning devices. Random mechanical motion can be lowered through the use of a high gain servo loop, but this leads to error compounding. A method for reducing servowriting errors arising from random mechanical motion to levels below that of the operating file TMR is highly desirable. As described above, servopattern errors larger than this increase the required space between data tracks, hence they result in lower disk file capacity. None of the prior art teaches about the problem of random mechanical motion resulting in reduced disk file capacity, or the relationship between random mechanical motion and error compounding, or even about error compounding by itself.